1. Field of the Invention
The present invention relates generally to hearing prostheses, and more particularly, to cancellation of bone-conducted sound in a hearing prosthesis.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. In some cases, a person may have hearing loss of both types. Conductive hearing loss occurs when the normal mechanical pathways that sound travels to the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss is often addressed with conventional hearing aids which amplify sound so that acoustic information can reach the cochlea.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. Sensorineural hearing loss occurs when there is damage to the inner ear or to the nerve pathways from the inner ear to the brain. Those suffering from sensorineural hearing loss are thus unable to derive suitable benefit from conventional hearing aids. As a result, hearing prostheses that deliver electrical stimulation to nerve cells of a recipient's auditory system have been developed to provide persons having sensorineural hearing loss with the ability to perceive sound. Such stimulating hearing prostheses include, for example, auditory brain stimulators and cochlear prostheses (commonly referred to as cochlear prosthetic devices, cochlear implants, cochlear devices, and the like; simply “cochlear implants” herein.) As used herein, a recipient's auditory system includes all sensory system components used to perceive a sound signal, including hearing sensation receptors, neural pathways, including the auditory nerve and spiral ganglion, and regions of the brain that sense sound.
Most sensorineural hearing loss is due to the absence or destruction of the cochlea hair cells which transduce acoustic signals into nerve impulses. It is for this purpose that cochlear implants have been developed. Cochlear implants use direct electrical stimulation of auditory nerve cells to bypass absent or defective hair cells that normally transduce acoustic vibrations into neural activity. Conventional cochlear implants have generally included an external assembly directly or indirectly attached to the body of the recipient, and an internal assembly which is implanted in the recipient.
More recently, cochlear implants have been designed such that all of the components of the device are implanted subcutaneously; that is there is no external assembly. Because such cochlear implants are entirely implantable, they are commonly referred to as a “totally” or “fully” implantable cochlear implant. One exemplary totally implantable cochlear implant is described in greater detail in U.S. Pat. No. 7,346,397.
Unlike conventional cochlear implants in which sound input elements, such as a microphone, are positioned external to the recipient, a totally implantable cochlear implant includes a subcutaneously-implanted microphone. An implanted microphone may be sensitive to airborne sound, referred to herein as air-conducted sound, as well as sound conducted to the microphone via the bones of the recipient's skull, referred to herein as bone-conducted sound.